This invention relates to an identification apparatus and an identification method for identifying an unknown system by the use of an adaptive filter. It is to be noted here that such an adaptive filter is used as an echo canceler, a noise canceler, a howling canceler, an adaptive equalizer, and the like to identify the unknown system, such as a transmission line, an audio coupling path, although the following description will be mainly directed to the echo canceler for canceling an echo which occurs in a two-wire/four-wire hybrid included in an echo path.
In general, an echo canceler is operated so that an echo which leaks from a transmission side to a reception side in a four wire side of a two-wire/four-wire hybrid is canceled by generating an echo replica corresponding to a transmission signal by the use of an adaptive filter which has taps of a number covering a time interval longer than an impulse response of an echo path. In the adaptive filter, each magnitude of tap coefficients is adaptively modified or updated by monitoring a correlation between the transmission signal and an error signal obtained by subtracting an echo replica signal from a mixture of the echo and a reception signal.
As an algorithm for adaptively modifying tap coefficients in the adaptive filter, an LMS algorithm and a Learning Identification Method; LIM are disclosed in articles which are contributed by B. Widrow et al to Proceedings of IEEE, Volume 63, No. 12, December, 1975, pages 1692-1716 (will be called Document 1 hereinunder) and contributed by J. Nagumo et al to IEEE Transactions on Automatic Control, VOL. AC-12, No. 3, June, 1967, pages 282-287 (will be referred to as Document 2 hereinafter), respectively.
Practically, it is noted that the impulse response often includes a fixed delay or a flat delay portion and a dispersive portion which forms a substantial impulse response waveform. The flat delay portion depends on a distance between a position of the echo canceler and a position of the two-wire/four-wire hybrid. In this event, it is necessary to prepare the taps of a number which covers a time interval of a predicted maximum flat delay portion and another time interval of a dispersive portion representative of a substantial impulse response. Accordingly, a long flat delay portion brings about necessity of a huge number of the taps, an increase of a hardware size, and a long convergence time resulting from a cross interference among the tap coefficients.
In order to solve the above-mentioned problems, an adaptive control method is proposed which estimates the dispersive portion on a time axis by removing the flat delay portion from the impulse response and by adaptively controlling an arrangement of tap coefficients in an adaptive filter only at a local region adjacent to an estimated position. Such a method is proposed in a paper which is contributed by S. Ikeda et al to Proceedings of International Conference on Acoustics, Speech, and Signal Processing, May, 1991, pages 1525-1528 and which is entitled "A Fast Convergence Algorithm for Adaptive FIR Filters with Coarsely Located Taps" (will be called Document 3). More specifically, a location of a dispersive portion is at first coarsely estimated and arranges tap coefficients of taps constrained in the vicinity of the estimated location. With this structure, it is possible to shorten a convergence time.
In the meanwhile, the location of the dispersive portion is estimated by the use of a maximum absolute value of tap coefficients. In this connection, a single range of the constrained tap alone is indicated to arrange the tap coefficients. Accordingly, when a plurality of dispersive portions or multiple echoes are present in an impulse response, the constrained tap range should be expanded so that all of the dispersive portions or multiple echoes are covered. This means that, when a large flat delay is present between the dispersive portions, a convergence time becomes long because an effect which results from constraining the taps is reduced.
Furthermore, proposal has been made about a method which enables a high speed convergence even in the presence of the multiple echoes by allocating tap coefficients only to dispersive portions. Such a method is disclosed in a paper which is published on A-9, pages 1-93 (will be called Document 4) by A. Sugiyama et al on Autumnal Meeting 1992 of Electronics, Information, Communication Engineers Institute in Japan.
According to this method, a tap control range which is specified by each of tap control subgroups is successively changed from one to another over a whole of the taps. Therefore, a comparatively high speed convergence can be accomplished even when the plurality of the dispersive portions, such as the multiple echoes, are present in the impulse response. As a result, tap coefficients can be arranged only at the dispersive portions of the impulse response at a high speed.
In the above-mentioned method mentioned in Document 4, the control tap subgroups are changed from one to another at a predetermined or uniform time interval irrespective of a degree of significance of each control tap subgroup. This shows that a long time is necessary for calculating tap coefficients of taps which need not calculate the tap coefficients. In other words, similar calculations should be carried out even when the taps have a small probability of calculating the tap coefficients. In consequence, a convergence time becomes long in the above-mentioned method.